1. Field of the Invention
This invention relates to a liquid phase polymerization process for producing a polyolefin elastomer, e.g., one derived from ethylene, another .alpha.-olefin such as propylene and, optionally, a diene, to a cation-generating cocatalyst for activating a metallocene procatalyst that can be employed in the polymerization process, to the resulting polyolefin elastomer possessing a desirably high molecular weight (M.sub.w), high Mooney viscosity (ML.sub.1+4 at 125.degree. C.), low polydispersity index (M.sub.w /M.sub.n), low glass transition temperature (T.sub.g) and low hysteresis (tan .delta.) and to various products manufactured therefrom including rubber articles such as hoses, belts and moldings, polymer blends containing one or more other hydrocarbon polymers and lubricating oils in which the elastomer functions as a viscosity modifier.
2. Description of the Prior Art
The most common polyolefin elastomers produced today are copolymers of ethylene and propylene (EP) and terpolymers of ethylene, propylene and a diene (EPDM). Ordinary EP elastomers can be cured using such curatives as organic peroxides, while the use of sulfur as a curative requires the incorporation of a diene. EPDM elastomers are usually produced with vanadium-organoaluminum catalysts, i.e., Ziegler-Natta catalysts.
Along with the better known EP and EPDM polymers, co- and terpolymers incorporating other .alpha.-olefins in place of propylene such as 1-butene, 1-pentene, 1-hexene, styrene, and combinations thereof are also known. EPDMs are representative of the more general category of ethylene-.alpha.-olefin diene elastomers (EODEs). Of the EODEs, EPDMs have achieved particular prominence due to the many properties which make them desirable for applications requiring good weather and acid resistance and high and low temperature performance. Notable applications of the EPDMs include their use in such products as hoses, gaskets, power transmission belts, conveyor belts, bumpers, automotive extrusions and moldings, weather stripping, blending components for plastics and rubbers such as polypropylene, polystyrene and butyl rubber, fabric coatings, viscosity modifiers for lubrication oils, tire sidewalls and in roofing and other membrane applications, shoe soles and heels and many other rubber articles. Another noteworthy application of the EPDMs is in wire and cable insulation due to their excellent dielectric properties.
It is desirable for an EPDM to have a reasonably fast cure rate and high state of cure, requirements calling for a relatively high diene content, e.g., three percent or higher. The cure rate for an EPDM elastomer and the final properties of the cured article depend upon the type of diene incorporated. For example, on a comparable diene weight percent basis, an EPDM produced with 5-ethylidiene-2-norbornene (ENB) as the diene will have a faster cure rate using a sulfur cure than would an EPDM containing dicyclopentadiene (DCPD) or 1,4-hexadiene (HD).
As for the properties of cured EPDM, EPDMs made with hexadiene as the termonomer are known to exhibit good heat resistance. For most commercial elastomer applications, the EPDM should have a weight-average molecular weight (M.sub.w) of at least about 300,000, or ML.sub.1+4 at 125.degree. C. of at least about 20 when expressed in terms of Mooney viscosity. In many applications, it is further desirable that the molecular weight distribution (MWD) of an EPDM be characterized by a ratio of weight average molecular weight to number average molecular weight (M.sub.w /M.sub.n), i.e., polydispersity index, of not greater than about 7 and preferably not greater than about 5.
The properties of an EPDM elastomer such as its tensile strength, processability and tack can be related to its degree of crystallinity. Since in most commercial uses elastomers are higher in molecular weight than plastics, too high a degree of crystallinity can make an EPDM difficult to process at ordinary temperatures. Although good physical properties are desirable, especially in such applications as hose, tubing, wire and cable, excessive crystallinity can cause an EPDM to exhibit high hardness and stiffness resulting in a "plastic" rather than a "rubber" surface with poor surface tack.
In general, commercially useful plastics, which are homo- and copolymers of ethylene, propylene, and higher .alpha.-olefins, need not have as high a molecular weight as commercially useful elastomers of ethylene-.alpha.-olefins such as EPDM. In terms of the catalysts used for each, when producing copolymers with compositions of M.sub.w in the elastomer range, catalysts that provide high M.sub.w plastic copolymers may produce low M.sub.w polymers unsuitable for elastomer applications. Similarly, undesirable MWD changes can occur or the compositional distribution can change. Thus, catalyst performance for the production of plastics is not indicative of catalyst performance for the production of elastomers.
In most current EPDM production, the catalysts conventionally employed in the production of high molecular weight EPDM elastomers are soluble vanadium catalysts such as VCl.sub.4, VOCl.sub.3, VO(Ac).sub.3 or VO(OR).sub.3 where R is an alkyl group together with an organoaluminum compound. The activity of the vanadium catalysts are relatively low, e.g., producing 5-20 kg polymer/g vanadium.
In current commercial grades of EPDM, crystallinity is a function of both the ethylene content of the polymer and the catalyst system used for its production. For a given polymer composition, the catalyst system controls the fraction of ethylene units present in long ethylene sequences which are capable of crystallizing. With any given catalyst and reactor configuration, polymers with higher ethylene content will have longer ethylene sequences and be more crystalline.
In current EPDM production based on vanadium catalysts, the product EPDM polymers are completely amorphous (non-crystalline) at ethylene contents below about 55 wt %. Conversely, at ethylene contents of about. 55 wt % or greater, an EPDM will possess significant crystallinity. The degree of crystallinity depends less on the diene content of the EPDM than on the percentage of ethylene.
In order for the catalyst system to be useful for the commercial production of an EPDM elastomer, it is desirable for the crystallinity of the polymer to be roughly comparable to that of currently available commercial grades of EPDM for most applications.
Metallocene catalysts typically consist of a transition-metal atom sandwiched between ring structures to form a sterically hindered site. Plastics obtained with metallocene catalysts tend to have increased impact strength and toughness, good melt characteristics, and improved clarity in films.
In actual practice, the extent to which metallocene catalysts can effectively replace traditional catalysts in polymer production depends on the cost and efficiency of the system. Metallocene catalysts cost significantly more than the traditional Ziegler-Natta catalysts but the metallocene systems are considerably more productive. In some cases, the increased productivity of metallocene catalysts relative to the Ziegler-Natta catalysts ranges from one to two orders of magnitude more polymer produced per pound of catalyst.
Since the recent introduction of aluminoxane-activated metallocene catalysts for the production of polyethylene, polypropylene, and copolymers of ethylene and .alpha.-olefins such as linear low density polyethylene (LLDPE), some effort has been made to apply these catalysts to the production of EPDM elastomers. For this use, it is desired that the catalyst produce high yields of EPDM in a reasonable polymerization time, result in adequate incorporation of the diene monomer(s) and provide a random distribution of monomers while enabling good control of M.sub.w over a wide range while yielding a relatively narrow MWD.
Kaminsky et al., J. Poly. Sc., Vol. 23, 2151-2164 (1985), discloses the use of a metallocene-methylaluminoxane (MAO) catalyst system to produce low molecular weight EPDM elastomers, i.e., M.sub.w s of not greater than about 150,000. Such catalysts require long reaction times and provide low yields and are therefore impractical for commercial EPDM manufacture. Similarly, Japanese Patent 62-121,771 describes a metallocene-catalyzed polymerization process yielding an ethylene-1-butene-diene elastomer of high ethylene content in low yield.
Other polymerization processes for producing EPDMs featuring the use of a metallocene catalyst activated by an aluminoxane such as MAO are described, e.g., in U.S. Pat. Nos. 4,871,705, 5,001,205, 5,229,478 and 5,442,020, EP 347,129 and WO 95/16716. As discussed more fully below, the lack of more widespread commercial implementation of metallocene catalysts where the production of high molecular weight elastomers is concerned is due at least in part to the need to use very large amounts of aluminoxane cocatalyst to activate the metallocene to acceptable levels.
EPA 593,083 describes a gas phase polymerization process for producing EPDM employing a bridged metallocene catalyst (1) ##STR1##
Gas phase polymerization, however, is prone to a number of technical difficulties, reactor fouling among them, that need to be overcome before this type of process for producing EPDM elastomers will achieve general acceptance by the industry.
EPA 612,769 and EP 653,445 both disclose the use of metallocene catalyst (1) in a solution phase polymerization process for producing linear low [molecular weight] propylene-diene elastomer (LLPDE) in contrast to a high molecular weight elastomer that is an object of the present invention.
U.S. Pat. No. 5,401,817 describes a polymerization process employing bridged metallocene catalyst (2): ##STR2##
There is, however, no mention of producing an elastomer in this patent.
Green et al., J. Chem. Soc. Dalton Trans., 657-665 (1994) describes the polymerization of propylene and styrene employing a bridged metallocene catalyst (3): ##STR3##
No mention of producing an elastomer is made in this publication.
Kaminsky et al., Angew. Chem. Int. Ed. Enql., 34, 2273-2275 (1995) describes bridged metallocene catalyst (4), together with MAO, for the copolymerization of ethylene with bulky cycloalkenes: ##STR4##
Another aspect of the present invention lies in the discovery that not all bridged metallocene-catalysts will provide high molecular weight elastomers. Thus, e.g., it has been found that bridged metallocene catalyst (5) ##STR5## which differs from metallocenes (1)-(4), supra, only in the nature of the bridging group joining the two cyclopentadienyl-derived ligands provides low molecular weight (&lt;50,000) ethylene-propylene copolymers. In contrast to this result and as discovered herein, activated metallocene catalyst (1) provides elastomers of high molecular weight (&gt;300,000).
Bridged metallocene catalysts (6) and (7) possessing the bis(indenyl) and the bis(fluorenyl) structures, respectively, are capable of providing high molecular weight amorphous ethylene-propylene copolymers: ##STR6## Metallocenes (6) and (7) are described in U.S. Pat. Nos. 5,145,819 (indenyl) and 5,436,305 (fluorenyl) for the production of homopolymers. No mention is made in either patent of employing the disclosed metallocene for the production of an EPDM-type elastomer.
As previously mentioned, it has been discovered that one of the obstacles to widespread commercial implementation of metallocene catalysis lies in the use of an aluminoxane as cocatalyst. Aluminoxanes are expensive and large amounts are required in order to activate the metallocene catalyst with which they are associated.